The present invention relates to a semiconductor laser apparatus used in optical communications and optical information processing and, more particularly, to a temperature-controlled semiconductor laser apparatus whose temperature can be controlled using a temperature controller and a temperature detection thermistor, and a temperature control method therefor.
A combination of a temperature controller using Peltier elements and a thermistor for detecting the temperature of a portion closest to a semiconductor laser mounting portion to perform temperature control of a semiconductor laser element is popularly used as a conventional temperature-controlled semiconductor laser apparatus. An arrangement used in a conventional temperature-controlled semiconductor laser apparatus will be described below.
FIG. 9 shows an arrangement of a conventional temperature-controlled semiconductor laser module. As shown in FIG. 9, a semiconductor laser element 1 and an island 5 are soldered (not shown) on a heat sink 23. The heat sink 23 is soldered (not shown) on a carrier 12. The thermistor 2 is soldered (not shown) on the carrier 12 near the semiconductor laser element 1.
A block 11 is soldered (not shown) to the carrier 12. A solder coating is formed on the outer surface of an optical fiber 6 and is fixed to the block 11 with a soldered portion 10 at a position where light emitted from the semiconductor laser element 1 is incident on the optical fiber 6.
Lead wires 17 of a temperature controller 18 are electrically connected to the lead terminals (not shown) of a package 7 to obtain external connections. The temperature controller 18 comprises Peltier elements 14, an upper substrate 15, a lower substrate 16, and the lead wires 17.
The basic principle of the temperature controller 18 will be described below. As shown in FIG. 11, n- and p-type semiconductor layers 27 and 28 are bonded between the upper substrate 15 and the lower substrate 16. The plurality of pairs of n- and p-type semiconductor layers 27 and 28 are connected to upper and lower soldered portions 29 in a staggered manner between the upper and lower substrates 15 and 16 to constitute a series connection circuit. The upper and lower substrates 15 and 16 normally consist of a material such as a ceramic material so as to achieve electrical insulation.
When a DC current flows in a direction of an arrow in FIG. 11, the upper substrate 15 side on which the current flows from the n-type semiconductor layer to the p-type semiconductor layer absorbs heat, while the lower substrate 16 side on which the current flows from the p-type semiconductor layer to the n-type semiconductor layer generates heat. The temperature controller 18 utilizes a Peltier effect by causing the current to flow through the two types of bonded semiconductor layers.
The lower surface of the lower substrate 16 of the temperature controller 18 is fixed with a soldered portion 19 on a base 30 of the package. The carrier 12 is fixed on the upper surface of the upper substrate 15 through a soldered portion 13. The semiconductor laser element 1 and the island 5 are electrically connected through wires 4.
The island 5 and lead terminals 22 are electrically connected through wires 9 and a wiring pattern 8, and the thermistor 2 and lead terminals 21 are electrically connected through lead wires 3 and a wiring pattern 20.
After the above components are mounted, a cap 32 is welded by seam welding and sealed by the package 7 in which nitrogen gas is kept sealed, as shown in FIG. 10, thereby obtaining the conventional temperature-controlled semiconductor laser apparatus.
Temperature control of the semiconductor laser element 1 in the conventional temperature-controlled semiconductor laser apparatus will be described below. The direction and magnitude of a current supplied to the lead wires 17 are controlled in accordance with a temperature detected by the thermistor 2. By this control, the temperature of the upper substrate 15 is controlled to keep the temperature of the semiconductor laser element 1 constant.
When the temperature of the semiconductor laser element 1 which is detected by the thermistor 2 is higher than the preset temperature of the semiconductor laser element 1, a current flows in the lead wires 17 of the temperature controller 18 in a direction to cool the semiconductor laser element 1. To the contrary, when the detected temperature is lower than the preset temperature, a current flows in a direction to heat the semiconductor laser element 1. When the difference between the detected temperature of the semiconductor laser element 1 and the preset temperature is large, the value of the current is controlled to increase. When the difference is small, the value of the current is controlled to decrease.
In the conventional temperature-controlled semiconductor laser apparatus, since the thermistor 2 is fixed on the carrier 12 near the semiconductor laser e element 1 on the temperature control target side of the temperature controller 18. For this reason, a space for fixing the thermistor 2 on the carrier 12 on the temperature control target side of the temperature controller 8 must be assured. The thermistor on the temperature control target side of the temperature controller 18 must be connected to the package 7 through the lead wires 3. For this reason, heat generated outside the package 7 is transferred to the temperature control target side of the temperature controller 18, and temperature control characteristics are greatly degraded.